Electric machines, such as electric motors or the like play a critical role in our society. They often provide operations which many times cannot be interrupted. Continued, uninterrupted operation is often critical to the devices for which they are a component. Oftentimes, they cannot be permitted to fail. Unfortunately, their operation can at times be attended by internal faults and weakened components. These weaknesses can exist in a stator, in a rotor or other moving component, or in the configuration of a rotating device.
Considering the stator of an induction machine as one example, it can be understood that the stator core of an induction machine is often built from thin insulated, silicon-steel (Si—Fe) laminations to minimize the hysteresis and eddy current losses for high efficiency operation. The individual lamination sheets are usually deburred and coated with insulation material to prevent conduction between the sheets to reduce the risk of inter-laminar eddy currents. The laminations are usually held tightly together and shorted to the frame for structural rigidity (prevention of vibration) and safety. The laminations may be welded together for small machines, and may be interlocked with the axial frame bars and clamped, for larger machines.
The stator core loss can account for roughly 20% of induction machine losses at rated load. It can increase due to any type of defect in the stator core caused by excessive thermal, electrical, mechanical, or environmental stresses. Since the interlaminar insulation material is subject to deterioration and damage, shorts between laminations can be introduced due to a number of reasons listed including:                Stator-rotor contact due to machine mechanical defects,        Core overheating during rewind (burnout oven), or due to ground current flow or cooling defects,        Defects, damage, or foreign materials introduced during manufacturing, inspection, or repair, and        Abrasion due to vibration (core looseness or machine vibration).If the laminations are shorted for any reason such as those listed above, a circulating interlaminar eddy current larger than that compared to normal operation can be induced in the core. The increase in eddy current can result in additional losses and can decrease the efficiency of the machine. Localized heating due to the fault current can also damage the stator winding insulation leading to ground failure. For large machines, the core fault can gradually progress in severity and can even result in melting of the stator core. In addition to the increase in eddy current losses, thermal stress or mechanical stress on the laminations due to compressive stress/physical abuse can increase the hysteresis losses. Exposure to excessive thermal or mechanical stresses can alter the magnetic properties of the lamination (decrease in permeability), and can result in an increase in hysteresis losses.        
When motors are operated with pulse width modulated (PWM) inverters, the stator core and inter laminar insulation can be exposed to higher thermal and electrical stresses. The high frequency voltage and current harmonic components due to PWM switching operation can cause higher capacitive leakage and can cause circulating eddy currents to flow in the stator core, which may result in additional heating. The cooling efficiency of the motor can also be reduced under low speed operation since the cooling fan speed can be reduced. This can result in operation at a higher temperature when the motor is driven with an inverter. The electrical (high dv/dt) and thermal (increased temperature) stresses can accelerate the degradation of the stator interlaminar insulation and core, and can increase the chances of failure. Although the above relates primarily to stator failures, it can be extended to rotor and other failures.
It can be seen from the discussion above that any type of defect in the stator core can increase the core losses (decrease in efficiency) and can increase the chances of machine failure (decrease in reliability). Since increase in core loss can be a symptom of stator core degradation leading to failure, it is important to monitor the core quality (losses) for reliable and efficient motor operation. It can also be seen that stator core condition monitoring is even more important for inverter-fed machines, since they are exposed to increased thermal and electrical stresses.
Unfortunately the available means of detecting core problems when the machine is in service has been to use chemical monitoring such as core monitors or tagging compounds to detect hot spots in the stator core. Although chemical monitoring is effective for detecting core problems on-line, it is only considered cost-effective for very large machines.
The most effective tests for detecting local damage in the stator core due to inter laminar insulation failure are the core ring test (loop test) and the low energy core tests. In the core ring test, an external winding is formed around the yoke of the core after rotor removal to excite the yoke of the stator core at 80˜100% of rated flux. After core excitation, an infrared IR thermal imaging camera is usually used to detect hot spots in the stator bore due to interlaminar fault currents. Low energy core tests can use the same excitation configuration, but may allow testing at 3˜4% rated flux level. In these tests, a flux sensing probe is scanned in the axial direction along the inner surface of the core to detect irregular flux patterns caused by inter laminar fault current. Although these tests are effective for detecting and locating local core problems, they require rotor removal and specialized test equipment. These tests are usually applied to generators or large motors rated above tens of megawatts. Tests may also use a probe such as described in a patent of one of the current inventors, U.S. Pat. No. 6,847,224, hereby incorporated by reference.
For motors rated up to several megawatts, which often fall in the range of inverter-fed induction machines, the core loss test is the most commonly used test for stator core quality assessment. The stator core loss test setup is often identical to the core ring test above, where the yoke of the stator core is excited at near rated flux (often ≈1.32 T). The power input (stator core loss) may be measured to monitor problems in the stator core and its insulation that result in an increase in core losses. This test can provide a good indication of the overall stator core condition, but it usually requires motor disassembly and rotor removal for testing.
The core loss can also be measured without motor disassembly if the loss segregation test procedure in IEEE STD. 112B is used. In this test method, the no load loss, which consists of the stator I2R loss, Ps, stator core loss, Pc,s and friction & windage loss, Pf&w, is measured at a number of input voltages, often up to about 125% rated voltage. If the no load loss minus Ps is plotted as a function of the voltage squared at a number of voltage levels and the curve is extrapolated to zero voltage, the intercept of the curve at zero voltage represents Pf&w. The stator core loss, Ps,c, can be obtained by subtracting Ps and Pf&w from the no load loss measurement. This test is also effective for monitoring the overall (average) condition of the core, but, it requires the motor to be operated at no load with a variable sinusoidal voltage source.
From the existing techniques, it can be seen that there are limitations to applying the currently available on-line and off-line core test methods to inverter-fed machines. On-line chemical monitoring cannot be justified for use in inverter-fed machines due to the cost involved in implementation. The off-line core ring test or low energy core tests could be used, but are not considered cost-effective for the machines rated below several megawatts. The core loss test is relatively simple and effective for monitoring core problems, but it usually can only be performed during a major outage since it requires machine disassembly and rotor removal. The no load loss segregation method does not require motor disassembly, but usually requires the motor to be run with the load removed at a number of voltage levels. It can be seen that all the core test methods are off-line tests that do not allow frequent monitoring of the core, and usually require motor disassembly or operation at no load. In addition, the tests suitable for inverter-fed machines can require specialized equipment for testing and may be only capable of testing the overall (or average) condition of the core.
Another type of fault is that of a rotor conductivity fault. Broken rotor bars (as one type of rotor conductivity fault), can account for 5˜10% of induction machine failures. These can be caused by gradual deterioration due to a combination of thermal, mechanical, electromagnetic, dynamic, environmental operating stresses or design/manufacturing defects. Often a crack is usually initiated between the bar and end ring connection. If a bar beaks, the rotor becomes electromagnetically asymmetric due to the absence of current in the broken bar.
There are both off-line and on-line methods available for detecting broken bars and some methods available include the single phase rotation test, growler test, magnetic imaging test, fluorescent dye penetration test, rated flux test, and the hot spot methods. The single phase rotation test is the only off-line method which does not require motor disassembly. In this test, though, two phases of the motor are excited with a single phase supply to produce a fluctuating field. The rotor is gradually rotated and the variation of the stator current is observed to detect the presence of rotor bars. This test is inconvenient because it requires proximity to the motor and the rotor shaft to be rotated, which could be very difficult depending on the motor and its operating environment.
The on-line methods analyze the spectrum of the speed, vibration, torque, flux, or current measurement to observe a particular speed dependent roto bar characteristic frequency to detect asymmetry in the flux pattern due to asymmetric rotor current. On-line methods are convenient since bars can be monitored when the motor is energized. However, the cost is high because dedicated monitoring equipment must be installed for each motor. Since it relies on the speed dependent frequency, a speed measurement or estimate is required and it is difficult to use in applications where the speed varies rapidly due to variable load or speed command. Broken bar signatures can also be confused with load problems, and it is difficult to detect this frequency for low slip (light load/high efficiency) applications. Therefore, it is desirable to have a new method that can detect broken bars off-line remotely without disassembling the machine. Thus, it is desirable to test both stator core and rotor components more frequently without motor disassembly, motor operation, or specialized equipment. It is also desired to have improved sensitivity to detect local core and other problems. Given the importance of stator core, rotor, or other monitoring for inverter-fed machines, and the limitations of applying the existing techniques, it is a goal of the present invention to provide new testing approaches suitable for inverter-fed machines and that may provide a sensitive stator core and rotor quality assessment technique that can be implemented for frequent monitoring without motor disassembly/operation, or additional hardware. Unfortunately none of the above techniques fulfill these criteria.